NAME

execve − execute a file

SYNOPSIS

execve(name, argv, envp)
char ∗name, ∗argv[], ∗envp[];

DESCRIPTION

Execve transforms the calling process into a new process.  The new process is constructed from an ordinary file called the new process file.  This file is either an executable object file, or a file of data for an interpreter.  An executable object file consists of an identifying header, followed by pages of data representing the initial program (text) and initialized data pages.  Additional pages may be specified by the header to be initialize with zero data.  See a.out(5).

An interpreter file begins with a line of the form “#! interpreter”; When an interpreter file is execve’d, the system execve’s the specified interpreter, giving it the name of the originally exec’d file as an argument, shifting over the rest of the original arguments. 

There can be no return from a successful execve because the calling core image is lost.  This is the mechanism whereby different process images become active. 

The argument argv is an array of character pointers to null-terminated character strings.  These strings constitute the argument list to be made available to the new process.  By convention, at least one argument must be present in this array, and the first element of this array should be the name of the executed program (i.e. the last component of name). 

The argument envp is also an array of character pointers to null-terminated strings.  These strings pass information to the new process which are not directly arguments to the command, see environ(7).

Descriptors open in the calling process remain open in the new process, except for those for which the close-on-exec flag is set; see close(2). Descriptors which remain open are unaffected by execve.

Ignored signals remain ignored across an execve, but signals that are caught are reset to their default values. The signal stack is reset to be undefined; see sigvec(2) for more information.

Each process has real user and group IDs and a effective user and group IDs.  The real ID identifies the person using the system; the effective ID determines his access privileges.  Execve changes the effective user and group ID to the owner of the executed file if the file has the “set-user-ID” or “set-group-ID” modes.  The real user ID is not affected. 

The new process also inherits the following attributes from the calling process:

process IDsee getpid(2)
parent process IDsee getppid(2)
process group IDsee getpgrp(2)
access groupssee getgroups(2)
working directorysee chdir(2)
root directorysee chroot(2)
control terminalsee tty(4)
resource usagessee getrusage(2)
interval timerssee getitimer(2)
resource limitssee getrlimit(2)
file mode masksee umask(2)
signal masksee sigvec(2)
affinitysee tmpaffinity(2)

When the executed program begins, it is called as follows:

main(argc, argv, envp)
int argc;
char ∗∗argv, ∗∗envp;

where argc is the number of elements in argv (the “arg count”) and argv is the array of character pointers to the arguments themselves. 

Envp is a pointer to an array of strings that constitute the environment of the process.  A pointer to this array is also stored in the global variable “environ”.  Each string consists of a name, an “=”, and a null-terminated value.  The array of pointers is terminated by a null pointer.  The shell sh(1) passes an environment entry for each global shell variable defined when the program is called. See environ(7) for some conventionally used names.

If a process has requested to be traced (via the ptrace(2) syscall), the execve will succeed only if the process has both read and write access to the file.  execve won’t actually write anything to the file, however. 

RETURN VALUE

If execve returns to the calling process an error has occurred; the return value will be −1 and the global variable errno will contain an error code. 

ERRORS

Execve will fail and return to the calling process if one or more of the following are true:

[ENOENT] One or more components of the new process file’s path name do not exist. 

[ENOTDIR] A component of the new process file is not a directory. 

[EACCES] Search permission is denied for a directory listed in the new process file’s path prefix. 

[EACCES] The new process file is not an ordinary file. 

[EACCES] The new process file mode denies execute permission. 

[ENOEXEC] The new process file has the appropriate access permission, but has a misformed or inconsistent header or bad magic number.  See a.out(5).

[ETXTBSY] The new process file is a pure procedure (shared text) file that is currently open for writing by some process. 

[ENOMEM] The new process requires more virtual memory than is allowed by the imposed maximum (getrlimit(2)), or the memory required to execute the new image exceeds the available space in the swap area.

[E2BIG] The number of bytes in the new process’s argument list is larger than the system-imposed limit of 10240 bytes. 

[EFAULT] Name, argv, or envp to an illegal address. 

[ENOENT] The path name is too long. 

[EIO] An I/O error occurred while reading from or writing to the file system. 

CAVEATS

If a program is setuid to a non-super-user, but is executed when the real uid is “root”, then the program has the powers of a super-user as well. 

SEE ALSO

exit(2), fork(2), execl(3), environ(7), a.out(5)

4BSD/DYNIX